In recent years, as the demand for photovoltaic power generation has increased, solar cells for that purpose have been developed, and various studies have been conducted to improve the light absorbing performance of light absorbing layers for solar cells, which are generally formed by sequentially disposing a back electrode layer, a light absorbing layer, a resistance buffer layer, and a transparent conductive layer on a substrate.
The light absorbing layer can be formed using a CIGS alloy, which is known to absorb wavelengths covering wide part of the spectral range of sunlight and have high ability to absorb light. Specifically, the light absorbing layer can be formed by sputtering onto a substrate such as a glass substrate using, as a sputtering target, a CIGS alloy composed of Cu, In, Ga, Se, and other elements.
Such sputtering for forming the light absorbing layer and other layers may be performed using a planar sputtering target bonded to a planar backing plate. In this case, annular part of the planar surface of the target is consumed, and the target surface region that can be used decreases, so that the surface cannot be effectively used. Therefore, in order to increase the efficiency of use of the target surface, so-called rotary sputtering technology has been put to use. In such technology, sputtering is performed using a cylindrical sputtering target, which is bonded to the outer surface of a cylindrical backing tube and rotated around its axis during the sputtering. FIG. 1 is a schematic diagram showing such a cylindrical sputtering target. FIG. 1 shows a cylindrical sputtering target 100 formed on the outer circumference of a cylindrical backing tube 101.
For example, as described in Patent Literature 1, conventional sputtering targets made of indium, including both planar and cylindrical types, are generally formed by a melt casting process that includes placing a backing plate or tube or other supporting substrate in a mold, pouring molten indium into the casting space of the mold in that state, in which the surface of the supporting substrate is exposed, and cooling and solidifying the molten indium.
In this melt casting process, the indium solidification rate during the cooling is difficult to keep constant over the whole of the casting space for forming the sputtering target. Particularly when a sputtering target with a length of more than 1 m is produced, the resulting sputtering target can have a non-uniform structure and coarse crystal grains. If sputtering is performed using such a target, it will be difficult to deposit a film with a sufficiently uniform thickness distribution on a substrate.
On the other hand, Patent Literature 2 shows that a tube-shaped sputtering target is deposited on a carrier tube by spraying molten indium onto the carrier tube being rotated. Concerning the use of such thermal spraying, Patent Literature 2 states that “The tubes thus produced had a fine-grained microstructure having a mean grain size of 50 to 500 μm depending on the process parameters. In most cases, the resulting mean grain sizes were less than 200 μm.” and that “Owing to the production of the sputtering target according to the invention in the form of layers, the microstructure is homogeneous across the thickness and jacket surface of the sputtering target.”